1. Field of the Invention
This invention relates to methods and apparatus for determining the magnetic track width of a magnetic head.
2. Description of the Related Art
A write head is typically combined with a magnetoresistive (MR) or giant magnetoresistive (GMR) read head to form a merged head, certain elements of which are exposed at an air bearing surface (ABS). The write head is made of first and second pole pieces having first and second pole tips, respectively, which terminate at the ABS. The first and second pole pieces are connected at the yoke by a back gap, whereas the first and second pole tips are separated by a non-magnetic gap layer. An insulation stack, which comprises a plurality of insulation layers, is sandwiched between the first and second pole pieces, and a coil layer is embedded in this insulation stack. A processing circuit is connected to the coil layer for conducting write current through the coil layer which, in turn, induces write fields in the first and second pole pieces. Thus, write fields of the first and second pole tips at the ABS fringe across the gap layer. In a magnetic disk drive, a magnetic disk is rotated adjacent to, and a short distance (fly height) from, the ABS so that the write fields magnetize the disk along circular tracks. The written circular tracks then contain information in the form of magneteeed segments with fields detectable by the read head.
An MR read head includes an MR sensor sandwiched between first and second non-magnetic gap layers, and located at the ABS. The MR sensor detects magnetic fields from the circular tracks of the rotating disk by a change in resistance that corresponds to the strength of the fields. A sense current is conducted through the MR sensor, where changes in resistance cause voltage changes that are received by the processing circuitry as readback signals. On the other hand, a GMR read head includes a GMR sensor which manifests the GMR effect In the GMR sensor, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in direction of magnetization in the free layer, which in turn causes a change in resistance of the GMR sensor and a corresponding change in the sensed current or voltage.
One or more merged heads may be employed in a magnetic disk drive for reading and writing information on circular tracks of a rotating disk. A merged head is mounted on a slider that is carried on a suspension. The suspension is mounted to an actuator which rotates the magnetic head to locations corresponding to desired tracks. As the disk rotates, an air layer (an xe2x80x9cair bearingxe2x80x9d) is generated between the rotating disk and an air bearing surface (ABS) of the slider. A force of the air bearing against the air bearing surface is opposed by an opposite loading force of the suspension, causing the magnetic head to be suspended a slight distance (flying height) from the surface of the disk.
One important parameter of a magnetic head is its magnetic track width. If a magnetic head has a narrow track width, the tracks along a magnetic disk can also be made narrow. If the tracks on the disk can be made narrow, additional tracks can be formed on the disk to thereby increase its storage capacity. Thus, much emphasis has been placed on making the track widths of magnetic heads as small as possible. In turn, therefore, quick and accurate methods are needed to determine the magnetic widths of magnetic heads with narrow track width sizes. At the present state-of-the-art, magnetic track width sizes are less than 0.3 xcexcm.
Conventional methods for determining the magnetic track width are either (1) quick but inaccurate or (2) accurate but slow, particularly when dealing with magnetic heads having narrow track widths. One conventional method determines the magnetic track width from a full track profile of a magnetic track written on a disk. The full track profile consists of a plurality of signal amplitudes read by the magnetic head across a track of a magnetic disk at a plurality of head positions. The full track profile generally forms a bell-shaped curve when graphed (head position along x-axis, signal level along y-axis). The full track profile magnetic write width MWWFTP may be obtained based on the difference in left and right head positions which read half of the maximum head signal amplitude. Although this method can be performed relatively quickly, it is only accurate when MWW greater than  greater than MRW (the magnetic read width) and when no side reading of the read sensor exists.
The off-track reading capability (OTRC), which is a measure of how far the read head can go off track without picking up interference from adjacent tracks, and erase band width (EBW) can be found using the well-known xe2x80x9ctriple-trackxe2x80x9d method. In this method, a particular track is selected on a disk and two adjacent tracks which surround this track are written to. The middle track is then subsequently written to at a different frequency than the adjacent tracks for a partial erasure. Next, the full track profiles from the adjacent tracks are obtained. Best-fit lines are then fitted on the right side of the left adjacent track profile and on the left side of the right adjacent track profile. The two head positions where these best-fit lines intersect the x-axis are identified, and the difference between these positions is the OTRC. This method also suffers from inaccuracy due to side reading error.
Another conventional method of determining the magnetic track width is the convolution method. In this method, the track width is determined by the convolution of the magnetic signal profile of the written track (assumed to be rectangular) and the micro-track width profile, based on
FTP(x)=∫R(xxe2x88x92y)MG(y)dy=MTP(xxe2x88x92y)MG(y)dy,
where R(x) is the reader response function, MG(x) is the magnetization of the data track, and FTP(x) and MTP(x) are the full and microtrack track profile, respectively. In FIG. 3, a graph 300 of a full track profile 302 and a microtrack profile 304 of the magnetic head is shown. In FIG. 4, a graph 400 of a microtrack profile 402 (which is the microtrack profile of FIG. 3 in a smaller scaling) depicts a mag-netic write width 404 and a magnetic read width 406 of profile 402. In this method, accurate results may be obtained despite the side-reading error. However, this method is too slow for use in production testing. Also, the off-track reading capability (OTRC) and erase band width (EBW) cannot be obtained using this method.
Accordingly, what is needed is a quick and accurate method for determining the magnetic track width of a magnetic head, especially for magnetic heads having very narrow track widths.
A quick and accurate method of determining the magnetic track width of a magnetic head is described herein. A full track profile of a magnetic track is obtained using the magnetic head. The full track profile includes a plurality of signal amplitudes read across a track of a magnetic disk at a plurality of magnetic head positions. Next, an initial track width value is determined from the full track profile data. Preferably, the initial value is the magnetic write width (MWWFTP) which is determined based on the difference in left and right head positions which read half of the maximum head signal level. This initial track width value is then adjusted with side reading correction values for determining the magnetic track width. The side reading correction values are based on an analysis of side reading xe2x80x9ctailsxe2x80x9d of the bell-shaped signal curve that is formed by the track profile data when graphed.
In one particular embodiment, the correction value for the left side reading tail (CSRL) is xcex94YL/aL and the correction value for the right side reading tail (CSRR) is xcex94YR/aR, respectively, such that the magnetic track width MWW=MWWFTPxe2x88x92CSRLxe2x88x92CSRR. The values aL and aR are slopes of best-fit lines fitted over left and right sides of the bell-shaped curve (YL=aL*Xoffset+bL and YR=aR*Xoffset+bR), respectively. The values xcex94YL and xcex94YR are obtained based on equations xcex94YL=AL(SL)xe2x88x92(∂AL+∂AR)/2 and xcex94YR=AR(SR)xe2x88x92(∂AL+∂AR)/2, respectively, where ∂AL=AL(SL)xe2x88x92AL(SLxe2x88x92X) and ∂AR=AR(SR)xe2x88x92AR(SR+X); SL and SR are head offset positions that reflect where the best-fit lines and the side reading tails begin to deviate; AL and AR are signal amplitudes at specified head positions; and X=(MWWFTPxe2x88x92MRW)/2.